The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive using an olefin block copolymer (OBC) to provide high initial bond resistance for making elastic components such as laminates containing elastic strands for use in disposable diapers.
The increasing complexity of manufactured goods, in particular disposable goods, also leads to major improvements and developments in the hot melt adhesive industry. Hot melt adhesives are being used to bond a wider variety of substrates, within a broader adhesive application process window, and for a large end-use portfolio. For example considering the diaper manufacturing industry, materials involved may be non-woven materials, polymeric films, and in general elastomeric components. These elastomeric components can be used in products like diapers, in a form of strands, films, nonwovens or any other continuous or discrete form.
Processability of hot melt adhesives are linked to their ability to be melted, and transported and/or coated in a molten stage to the final location where the bond is required. Usually the molten adhesive is sprayed, or coated as a film. Once cooled down, the adhesive needs to fulfill multiple requirements, like bond strength measured by peel force or bond retention under or after mechanical stress, and under or after various thermal conditions.
Typically hot melt adhesives can be based on polymers such as polyolefins (ethylene- or propene-based polymers), or functionalized polyolefins (ethylene or propene copolymers with oxygen containing monomers), or styrenic block copolymers containing at least one rubbery phase, like styrene-isoprene-styrene (SIS), or styrene-butadiene-styrene (SBS) polymers. Styrenic block copolymers are of interest due to their dual characteristics, i.e. cohesion of the styrenic phase associated with the rubber behavior of another phase. Typical application temperatures are equal to or higher than 150° C.
Over the years, many different olefinic polymers have been used in the formulation of hot melt adhesives used in the construction of disposable soft goods. The first of these was amorphous polypropylene (APP). This material was produced as a by-product of crystalline polypropylene and was obtained by solvent extraction. This APP polymer could be combined with various tackifiers, plasticizers, waxes, etc. to produce a hot melt that could be used for diaper construction, for example.
Later, polymers became available that were produced on purpose that had much improved properties over the original APP polymers. These were referred to as amorphous poly alpha olefins (APAO). They were primarily produced using Ziegler-Natta catalysis and could be made using a variety of monomers, including but not limited to propylene, ethylene and butene. Various copolymers and terpolymers are produced by a number of manufacturers. They include Evonik Industries, who produce the Vestoplast® polymers; REXtac, LLC, who produces the Rextac® RT range of materials and Eastman Chemical, manufacturers of the Eastoflex® line of polymers. They are all characterized by having a very low degree of crystallinity as measured by DSC. As commercially produced, they are random polymers having broad molecular weight distributions.
When formulated into hot melt adhesives for the construction of disposable articles, they had some deficiencies. They generally lacked elevated temperature heat resistance (particularly creep resistance) so they were not used for elastic attachment. This was due to their amorphous character. While they found widespread use for the diaper construction application (bonding the nonwoven to the polyethylene) they did not possess the level of elevated temperature creep resistance needed for the elastic attachment application.
One other reason APAO based hot melt adhesives were not used for elastic attachment was their poor sprayability. While the construction application is applied in a variety of ways, the elastic adhesive is almost always applied using spray application equipment. Compared to the construction application, the spray application for elastic attachment is must more demanding. The adhesive is generally applied hotter and at a higher add-on level than the construction application. This can lead to burn-through problems if not properly applied. In addition, the elastic application needs to be applied more precisely, that is directly onto the strands of elastic, instead of an overall construction application.
Historically traditional polyolefins such as polyethylene or polypropylene have not been used for any diaper construction applications. While these polymers are used in hot melt adhesives for packaging applications (e.g. case and carton sealing), they lack the adhesion, open time and sprayability needed for disposable applications. Examples of these types of polymers include the Epolene® polymers from Westlake Chemical Company.
More recently, metallocene catalysis has been used to make polyolefins with more precisely tailored properties. For example, the molecular weight of the polymer can be controlled in a way not possible with the older Ziegler-Natta catalysts. Polymers can be made using high levels of comonomer, such as butene-1 and octene-1, to produce polymers with very low levels of crystallinity and density. While these polymers have been used to make hot melt adhesives with better adhesion characteristics, they have not been widely used in the Nonwovens industry because of their lack of sprayability. Examples of these metallocene polymers include Affinity® and Engage® polymers from Dow Chemical Company.
The standard in the disposable industry in terms of sprayability have been hot melts based on styrenic block copolymers, specifically styrene-isoprene-styrene (SIS) block copolymers. No olefinic based polymer has been able to match the characteristics of the styrenic block copolymers in terms of ease of sprayability and application window. The term “application window” means the range of conditions a given adhesive will apply well. For example, if a given hot melt adhesive can only be applied over a narrow range of temperatures, flow rates, air pressures, open times, etc. it is described as having a narrow application window. If on the other hand an adhesive can be applied over a wide range of conditions and still give acceptable bonds, it is described as having a broad application window. It is very important that products used in the manufacture of disposable goods have a broad application window to minimize downtime and scrap during line speed fluctuations that occur during line start-up for example, or temperature fluctuations that might happen during production. Since these manufacturing lines frequently operate at line speeds over 1000 feet per minute, it is important to minimize scrap.
Polyolefin polymers are produced in a very wide range of molecular weights, monomers, densities and crystallinity levels. They are also made using an ever widening range of catalysts. There are Ziegler-Natta catalysis, metallocene and other single cite catalysts and more recently those that can produce block polyolefins.
These polymers range in crystallinity from very low, such as with amorphous polypropylene or amorphous poly-alpha-olefins to those that are very high, such as isotactic polypropylene. The crystallinity of a polymer can be determined by Differential Scanning Calorimetry (DSC) or X-ray Diffraction techniques. DSC is the most widely used technique by far. The Enthalpy of Fusion (also known as latent heat of melting or heat of fusion) can be measured and quantified using ASTM E793-01 entitled “Standard Test Method of Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry”. The enthalpy of fusion is the amount of energy it takes to melt the crystalline portion of the polymer. This value is generally reported in Joules/gram (J/g).
This number varies widely from almost zero to upwards of 250 Joules/gram depending on the crystallinity of the polymer. Ideally, a truly amorphous polymer would have no crystallinity, no melting point and therefore an enthalpy of fusion of zero. As it states in U.S. Pat. No. 7,524,911 (column 8, lines 30-33), “The term ‘amorphous’ refers to a polymer lacking a crystalline melting point as determined by differential scanning calorimetry (DSC) or equivalent technique”.
As a practical matter, most polymers that are sold as “amorphous poly-alpha-olefins” (APAO) have some low level of crystallinity. On the other hand, polymers that are considered crystalline are not 100 percent crystalline. In the '911 patent it states at column 8, lines 26-30, “The term ‘crystalline’ refers to a polymer that possesses a first order transition or crystalline melting point (Tm) as determined by differential scanning calorimetry (DSC) or equivalent technique, and this term may be used interchangeably with the term ‘semicrystalline’.”
It is useful to have some quantifiable boundary between what is considered “amorphous” polymer and those considered “semi-crystalline” or “crystalline”. In U.S. Pat. No. 6,747,114 it states at column 8, lines 9-14, “The semi-crystalline polymer preferably has a heat of fusion from about 30 J/g to about 80 J/g as determined by DSC, more preferably from about 40 J/g to about 70 J/g as determined by DSC, and most preferable from about 50 J/g to about 65 J/g as determined by DSC.”
Bostik's internal analysis correlates with the descriptions above. The “amorphous poly-alpha olefins” are not in fact entirely amorphous and possess a very low level of crystallinity as measured by DSC. The analysis of many of the grades sold by Eastman Chemical Co. as “Amorphous Polyolefins” under the trade name Eastoflex® and those sold by Evonik Industries as “Amorphous Poly-alpha-olefins” under the trade name Vestoplast® and those manufactured by REXtac, LLC. as REXtac RT show that all of them have an enthalpy (or heat) of fusion of less than 25 Joules/gram. The single highest value obtained was 20.4 Joules/gram for Vestoplast® 708. One of the two grades shown in U.S. Pat. No. 7,517,579 (assigned to Kimberly-Clark Worldwide, Inc.) is RT2730, which has a heat of fusion of 9.4 Joules/gram. The other grade that is mentioned is RT2723, which according to REXtac's usual nomenclature should be a lower viscosity version of RT2730 with the same monomer ratios. Therefore, the enthalpy of fusion should be similar to RT2730. In summary, currently available data strongly indicates that any grade of polymer currently sold as an “amorphous poly-alpha-olefin” would have an enthalpy of fusion value of less than about 25 Joules/gram.
A wide range of other polyolefins are produced by a variety of manufacturers that fall under the category of “semi-crystalline” polymers. They have heat of fusion values of greater than about 30 Joules/gram, which puts them outside the range of APAO's. For example, ethylene vinyl acetate copolymers range from about 35 Joules/gram for a high vinyl acetate grade (40% VA) to about 73 Joules/gram for a lower vinyl acetate grade (18% VA). Polyalphaolefins such as Dow's Affinity® grades (ethylene/octane copolymers) range from about 52 Joules/gram for Affinity® 8200, a relatively low density grade (0.870 g/cc, MI=5) to 77 J/g for a higher density grade (0.900 g/cc, MI=6) called Affinity® PL 1280. Dow also manufacturers a high melt index grade (0.870 g/cc, MI=1000) called GA1900 specifically for hot melt adhesives that has a heat of fusion of 57 Joules/gram. Clearly, these Affinity® polymers could not be considered to be amorphous and are not amorphous poly-alpha-olefins.
A more recent development in the area of polyolefins is what are referred to as “olefin block copolymers” or OBC. This is an entirely new class of polyolefin polymer produced using a chain shuttling catalysis technology that produces a linear block structure of the monomers rather than a random polymer produced by Ziegler-Natta or traditional metallocene technology. At this time, they are manufactured by Dow Chemical under the trade name of Infuse®. The OBC's consist of crystallizable ethylene-octene blocks (hard) with very low comonomer content and high melting temperature alternating with amorphous ethylene-octene blocks (soft) with high comonomer content and low glass transition temperature. This gives the polymer much better elevated temperature resistance and elasticity compared to a typical metallocene random polymer of similar density. While some of the grades of Infuse® have low heat of fusion (approximately 20 Joules/gram) they could not be considered to be amorphous poly-alpha-olefins because the polymer architecture is completely different (i.e. block vs. random) and is specifically produced to have crystalline regions. Not only are they different on a structural basis, they are very different from a physical property standpoint with the OBC's having better elastic recovery, compression set and elevated temperature resistance. As such, they are sold into different markets for different end uses and are not considered equivalent for one another.
U.S. Pat. No. 7,524,911 and WO 2009/029476 disclose adhesive compositions based on olefin block copolymers (OBC). Other references disclosing OBC's and various applications for OBC's include WO 2006/101966, WO 2006/102016, WO 2008/005501, and WO 2008/067503.